Here's the droid you are looking for.... (rear seat spring)
ZigZag Upholstery Springs - $1.99 each (You need 8; 4 per seat)
Available from DIY Upholstery
Part number SIN20
As I mentioned, you will need to bend the springs into the angle that the seat needs for tension. I have some pictures of mine, somewhere.
This isnt the best shot, but it's all I have for now. You can see the bends in the spring here. You do NOT want to leave the springs "flat." I still have my originals if you need me to measure where to begin the bend. (I painted mine flat black.)